Intrusion detection devices have been available for decades and more than two-dozen types of intrusion detection technologies are currently in use. In general, the performance and cost of indoor intrusion detection devices are satisfactory. However, outdoor intrusion detection devices are far from desirable in terms of both performance and cost. The environmental factors and detection distance in an outdoor setting are the two main challenges. For example, a passive infrared motion detector, currently considered to be one of the most common types of intrusion detection device, has limited effectiveness for outdoor applications due to its limited detection distance and high false alarm rate in an outdoor environment.
For outdoor detection where significant distances (such as longer than 40 feet) are involved, photoelectric beams and microwave detectors are more commonly used as they have lower false alarm rates and longer detection range. For distance more than a few hundred feet, bistatic units (separate transmitter and receiver units) are usually required for both photoelectric beams and microwave. Good alignment of the transmitter and receiver is critical, which makes installation more difficult and the system more vulnerable to defeat. Installing intrusion detection cables around a premise is also an option, but the installation costs of the cable and the required supporting electronics are very high. For very long-range outdoor intrusion detection, radar is sometimes used, but it is even more costly.
Even if one sets aside important issues such as cost, vulnerability to defeat, and probability of detection, none of the above mentioned technologies have a perfect false alarm rate. Despite the incorporation of sophisticated electronics, they cannot ascertain with certainty if an intrusion alarm is a real threat or a false alarm. Additional visual verification via a camera or in person is almost always necessary.
There are products available that combine detection and visual verification into one unit. For short-range applications, combining a passive infrared sensor and a video camera into one housing is increasingly common. When the passive infrared sensor detects motion, the video camera is activated to capture the range of a scene for threat evaluation. In a similar way, any long-range intrusion detection device can be combined with a camera for threat evaluation as well. Alternatively, video motion detection can be used, wherein a camera's video image is utilized to detect motion and threat assessment at the same time. When the video captured by a camera detects motion, one or more alarms are triggered, which may include transmitting of the images for threat assessment. However, the false alarm rate of video motion detection is too high for most intrusion detection applications.
With the advent of microprocessors in the 1980s, video motion detection became more viable. Further advances on more sophisticated motion detection algorithms were made in the 1990s. In recent years, even low cost surveillance systems or cameras have built-in video motion detection capability.
Although the false motion alarm rate has been improved substantially, as stated above, it is still rather high. Moreover, in dark environments such as nighttime surveillance, motion detection is extremely difficult unless adequate light is added to increase visibility. In many situations, adding light is not desirable or feasible. For the above reasons, most video motion detection is used mainly for triggering the start of video recordings or transmission. The security industry's interest in video motion detection peaked in the late 1990s and a very limited number of new products have been introduced in the past decade.
A thermal security camera can overcome the night vision problems of conventional video cameras as it can “see” in total darkness through sensing heat, not light. This characteristic makes a thermal security camera a good video motion detection device, except that the cost is extremely high. Even the most affordable low-resolution thermal camera costs many thousands of dollars. Further, their false alarm rates are as high as conventional cameras. In addition, thermal cameras cannot see the details of a scene, and the images are always in monochromatic format.